For goals like a higher speed of operation and a saving of power consumption, a challenge to higher integration of large-scale integrated circuits continues. To meet increasing demands for shrinkage of circuit patterns, the advanced semiconductor microprocessing technology becomes important. For example, the technology for shrinkage of circuit-constructing wiring patterns and the technology for shrinkage of contact hole patterns for cell-constructing inter-layer connections become essential.
The advanced microprocessing technology relies on the photolithography using photomasks. The photomask is one important area of the miniaturization technology as are the exposure tool and resist material. To obtain a photomask capable of affording a fine-size wiring pattern or fine-size contact hole pattern as mentioned above, efforts are made to develop the technique of forming a more fine and accurate pattern on a photomask blank.
In order to form a high accuracy photomask pattern on a photomask substrate, it is of first priority to pattern a resist film on a photomask blank at high accuracy. Since the photolithography for microprocessing semiconductor substrates employs reduction projection, the size of a pattern formed on a photomask is about 4 times the size of a pattern formed on a semiconductor substrate, which does not mean that the accuracy of the pattern formed on the photomask is accordingly loosened. It is necessary that the photomask pattern be formed at a high accuracy.
At the present, the size of a circuit pattern written on a semiconductor substrate by photolithography is far smaller than the wavelength of exposure light. If reduction exposure is carried out using a photomask having a pattern which is a mere 4-time magnification of the circuit pattern, the photomask pattern is not faithfully transferred to the resist film due to impacts such as interference of exposure light.
Super-resolution masks addressing the problem include OPC masks in which the so-called optical proximity correction (OPC), i.e., the technology for correcting the optical proximity effect to degrade transfer properties is applied to photomasks and phase shift masks which cause a phase shift of 180° between adjacent pattern features to establish a sharp intensity distribution of incident light. For example, in some OPC masks, an OPC pattern (hammer head, assist bar or the like) having a size of less than half of a circuit pattern is formed. The phase shift masks include halftone, Levenson and chromeless types.
In general, a photomask pattern is formed by starting with a photomask blank having a light-shieldable film on a transparent substrate, forming a photoresist film on the photomask blank, exposing the photoresist film to light or electron beam (EB) to write a pattern, and developing the photoresist film to form a photoresist pattern. Then, with the photoresist pattern made mask, the light-shieldable film is etched to form the photomask pattern. To obtain a fine photomask pattern, it is effective to reduce the thickness of a photoresist film (i.e., thinner resist film) for the following reason.
If only a resist pattern is shrunk without reducing the thickness of a resist film, the resist pattern functioning as the etching mask for the light-shieldable film has a higher aspect ratio (ratio of resist film thickness to pattern width). In general, as the aspect ratio of resist pattern becomes higher, the pattern profile is more likely to degrade. Then the accuracy of pattern transfer to the light-shieldable film via the resist pattern as the etch mask is reduced. In extreme cases, the resist pattern partially collapses or strips off, resulting in pattern dropouts. In association with the shrinkage of a photomask pattern, it is necessary that the resist film used as the etching mask during patterning of a light-shieldable film is thinned to prevent the aspect ratio from becoming too high. An aspect ratio of up to 3 is generally recommended. To form a resist pattern having a feature width of 70 nm, for example, a resist film thickness of up to 210 nm is preferable.
For the light-shieldable film which is etched using the pattern of photoresist as an etch mask, on the other hand, a number of materials have been proposed. In particular, neat chromium films and chromium compound films containing chromium and at least one of nitrogen, oxygen and carbon are generally used as the light-shieldable film material. For example, Patent Documents 1 to 3 disclose photomask blanks wherein chromium compound films are formed as the light-shieldable film having light shielding properties necessary for the photomask blank for use in ArF excimer laser lithography.
The light-shieldable film in the form of chromium compound film is generally patterned by oxygen-containing chlorine dry etching, during which an organic film, typically photoresist film can be frequently etched to a noticeable extent. If the light-shieldable film in the form of chromium compound film is etched with a relatively thin resist film made mask, the resist is damaged during the etching so that the resist pattern is deformed. It is then difficult to accurately transfer the resist pattern to the light-shieldable film.
The attempt to endow the photoresist or organic film with high resolution and high patterning accuracy as well as etch resistance encounters a technical barrier. The photoresist film must be reduced in thickness for the goal of high resolution whereas thinning of the photoresist film must be limited for the purpose of ensuring etch resistance during etching of the light-shieldable film. As a result, there is a tradeoff relationship between high resolution/patterning accuracy and etch resistance.
To mitigate the load to the photoresist to enable film thickness reduction for eventually forming a photomask pattern of higher accuracy, the construction (including thickness and composition) of a light-shieldable film to be patterned must be ameriolated.
As to light-shieldable film materials, a number of studies have been made. For example, Patent Document 4 discloses a metal film as the light-shieldable film for ArF excimer laser lithography. Specifically, tantalum is used as the light-shieldable film and tantalum oxide used as the antireflective film. To mitigate the load applied to the photoresist during etching of these two layers, the layers are etched with a fluorine base gas plasma which causes relatively few damages to the photoresist. Even though such etching conditions are chosen, when two layers, light-shieldable film and antireflective film are etched using only the photoresist as etch mask, the mitigation of the load to the photoresist is limited. It is difficult to fully meet the requirement to form a fine size photomask pattern at a high accuracy.
As discussed above, the prior art photomask blank structure is difficult to fully meet the requirement to form a fine size photomask pattern on the light-shieldable film at a high accuracy. The problem becomes more serious with the photolithography using exposure light of shorter wavelength and requiring higher resolution, typically light with a wavelength of up to 200 nm (e.g., ArF excimer laser 193 nm, F2 laser 157 nm).
As the light-shieldable film exhibiting a high etch rate during chlorine base dry etching that enables to mitigate the load to the photoresist for eventually forming a fine size photomask pattern at high accuracy, Patent Document 5 describes a light-shieldable film based on chromium and having light elements O and N added thereto, and Patent Document 6 describes a chromium compound film based on chromium and having a low melting metal such as Sn or In added thereto.
On the other hand, defect inspection on a photomask blank is generally carried out based on reflection by the blank. To detect defects of microscopic size, inspection light of shorter wavelength must be used. Light of wavelength 257 nm is currently used. To ensure accurate defect inspection on photomask blanks for ArF excimer laser lithography, a reflectance of the order of 10 to 20% with respect to light of this wavelength is necessary.
Nevertheless, a film of light element-containing chromium compound tends to have an increased transmittance and a reduced reflectance in the wavelength region of at least 200 nm. On alignment of a photomask utilizing transmitted or reflected light in the wavelength region of at least 400 nm for the reading of an alignment mark, there arises a problem of unstable alignment. A thick film is used to gain a necessary optical density, but is disadvantageous for reducing the feature size.
As the content of light elements increases, the light-shieldable film becomes less conductive. In the advanced technology where it is critical to reduce the feature size of a pattern transferred from a photomask to a wafer, the EB writing now takes the mainstream position in patterning a resist film during the manufacture of a photomask, as a replacement for the laser beam writing. For EB emission, a high accelerating voltage of 50 keV is employed in order to enable further miniaturization. There is a tendency of reducing the sensitivity of resist to achieve a higher resolution, while a remarkable leap of the current density from 40 A/cm2 to 400 A/cm2 is attempted from the aspect of productivity enhancement. Thus, the chromium compound film whose light element content is increased to gain a higher etch rate experiences a charge buildup during exposure of resist to EB, inviting a loss of imaging accuracy (increase of CD, shift of imaging position).